Why was "tantrum" trending last week?
1550s, "darkness of meaning, obscurity," from French opacité, from Latin opacitatem (nominative opacitas) "shade, shadiness," from opacus "shaded, dark, opaque" (see opaque). The literal sense "condition of being impervious to light" first recorded 1630s.
opacity o·pac·i·ty (ō-pās'ĭ-tē)
The quality or state of being opaque.
An opaque or nontransparent area, as of the cornea.